When you're adding a new account to Outlook, it suggest the previously configured email addresses.
If you want to remove an account from that suggestion, you should follow the next steps:
First, you should remove the accounts from your computer, go to Windows Settings > Accounts and remove the unwanted accounts.
Next, go to the following folder and remove the app_identity_provider from the unwanted accounts.
C:\Users\%username%\AppData\Local\Microsoft\OneAuth\blobs
Finally, go to this folder and remove the files containing the information about the unwanted accounts.
C:\Users\%username%\AppData\Local\Microsoft\OneAuth\accounts
Now Outlook should only show the account suggestion you decided to keep.